The present invention is directed to novel chromium-based catalysts and to an improved process for producing polyolefins, in particular polyethylene, by contacting an olefin monomer with the catalysts of the subject invention in a reaction zone, typically a single reaction zone, to provide a resultant polyolefin that exhibits bimodal molecular weight distribution.
In the field of polyolefin manufacture, much attention has been devoted to finding new and improved catalysts capable of producing polyolefins having unique and/or improved properties or capable of providing such polyolefins in a more economical manner.
The best known industrially used catalyst systems for the polymerization of olefins are those of the xe2x80x9cZiegler-Natta catalystxe2x80x9d type and the xe2x80x9cPhillips catalystxe2x80x9d type. The former comprises the reaction product of a metal alkyl or hydride of elements of the first three main groups of the Periodic Table and a reducible compound of a transition metal element of Groups 4 to 7. The combination used most frequently comprising an aluminum alkyl, such as diethylaluminum chloride, and titanium (IV) chloride. More recently, highly active Ziegler-Natta catalyst systems have been formed in which the titanium compound is fixed chemically to the surface of magnesium compounds, such as, in particular, magnesium chloride.
The Phillips Process for ethylene polymerization developed around Phillips catalyst that is composed of chromium oxide on silica as the support. This catalyst was developed by Hogan and Banks and described in U.S. Pat. No. 2,825,721, as well as A. Clark et al. in Ind. Eng. Chem. 48, 1152 (1956). Commercialization of this process provided the first linear polyalkenes and accounts for a large amount of the high-density polyethylene (HDPE) produced today.
More recent developments have focused on single-site catalyst systems. Such systems are characterized by the fact that their metal centers behave alike during polymerization to make very uniform polymers. Catalysts are judged to behave in a single-site manner when the polymer they make meets some basic criteria (e.g., narrow molecular weight distributions, or uniform comonomer distribution). Thus, the metal can have any ligand set around it and be classified as xe2x80x9csingle-sitexe2x80x9d as long as the polymer that it produces has certain properties. Includable within single-site catalyst systems are metallocene catalysts, and constrained geometry catalysts.
A xe2x80x9cmetallocenexe2x80x9d is conventionally understood to mean a metal (e.g., Zr, Ti, Hf, Sc, Y, Vi or La) complex that is bound to two cyclopentadienyl (Cp) rings, or derivatives thereof, such as indenyl, tetrahydroindenyl, fluorenyl and mixtures. In addition to the two Cp ligands, other groups can be attached to the metal center, most commonly halides and alkyls. The Cp rings can be linked together (so-called xe2x80x9cbridged metallocenexe2x80x9d structure), as in most polypropylene catalysts, or they can be independent and freely rotating, as in most (but not all) metallocene-based polyethylene catalysts. The defining feature is the presence of two Cp ligands or derivatives thereof.
Metallocene catalysts can be employed either as so-called xe2x80x9cneutral metallocenesxe2x80x9d in which case an alumoxane, such as methylalumonxane, is used as an activator or they can be employed as so-called xe2x80x9ccationic metallocenesxe2x80x9d which incorporate a stable and loosely bound non-coordinating anion as a counter ion to a cationic metal metallocene center. Cationic metallocenes are disclosed in U.S. Pat. Nos. 5,064,802; 5,225,500; 5,243,002; 5,321,106; 5,427,991; and 5,643,847; and EP 426 637 and EP 426 638.
xe2x80x9cConstrained geometryxe2x80x9d is a term that refers to a particular class of organometallic complexes in which the metal center is bound by only one modified Cp ring or derivative. The Cp ring is modified by bridging to a heteroatom such as nitrogen, phosphorus, oxygen, or sulfur, and this heteroatom also binds to the metal site. The bridged structure forms a fairly rigid system, thus the term xe2x80x9cconstrained geometry.xe2x80x9d By virtue of its open structure, the constrained geometry catalyst can produce resins (long chain branching) that are not possible with normal metallocene catalysts.
The above-described single site catalyst systems are primarily based on early transition metal d0 complexes useful in coordination polymerization processes. However, these catalysts are known to be oxophilic and, therefore, have low tolerance with respect to even small amounts of oxygenated impurities, such as oxygen, water and oxygenated hydrocarbons. Thus, these materials are difficult to handle and use.
More recently, late transition metal (e.g., Fe, Co, Ni, or Pd) bidentate and tridentate catalyst systems have been developed. Representative disclosures of such late transition metal catalysts are found in U.S. Pat. No. 5,880,241 and its divisional counterparts, U.S. Pat. Nos. 5,880,323; 5,866,663; 5,886,224; 5,891,963; 6,184,171; 6,174,976; 6,133,138; and PCT International Application Nos. PCT/US98/00316; PCT/IJS97/23556; PCT/GB99/00714; PCT/GB99/00715; and PCT/GB99/00716.
For polyethylene, and for high density polyethylene (HDPE) in particular, the molecular weight distribution (MWD) is a fundamental property which determines the properties of the polymer, and, thus, its applications. It is generally recognized in the art that the molecular weight distribution of a polyethylene resin can principally determine the physical, and in particular, the mechanical properties of the resin. Further, the provision of different molecular weight polyethylene molecules can significantly affect the Theological properties of the polyethylene as a whole.
Since an increase in the molecular weight normally improves the physical properties of polyethylene resins, there is a strong demand for polyethylene having high molecular weight. However, it is the high molecular weight molecules that render the polymers more difficult to process. On the other hand, a broadening or preferably a bimodal molecular weight distribution tends to improve the flow of the polymer when it is being processed at high rates of shear. Accordingly, in applications requiring a rapid transformation resulting in high expansion of the material through a die, for example in blowing and extrusion techniques, having a bimodal molecular weight distribution permits an improvement in the processing of polyethylene at high molecular weight (this being equivalent to a low melt index, as is known in the art). It is known that when the polyethylene has a high molecular weight and also a bimodal molecular weight distribution, the processing of the polyethylene is made easier as a result of the low molecular weight portion and also the high molecular weight portion contributes to a good impact resistance for the polyethylene film. A polyethylene of this type may be processed utilizing less energy with higher processing yields.
It is known in the art that it is not practical to prepare a polyethylene having a bimodal molecular weight distribution and the required properties simply by mixing polyethylenes having different molecular weights.
As discussed above, high-density polyethylene consists of high and low molecular weight fractions. The high molecular weight fraction provides good mechanical properties to the high density polyethylene and the low molecular weight fraction is required to give good processability to the high density polyethylene, the high molecular weight fraction having relatively high viscosity which can lead to difficulties in processing such a high molecular weight fraction.
On the other hand, a bimodal molecular weight distribution provides a composite of high and low glass transition temperature weight fractions. Such bimodal weight distribution polymer composition provides desired material that exhibits good processability while providing a tough, resilient material which, as an example, exhibits high environmental stress cracking resistance (ESCR). The ESCR of polymers desirably will be greater than 200 hours, such as 500 hours or more and more preferably from about 1000 to 10,000 hours when tested under known procedure ASTM D-1693-70, Condition B.
It is accordingly recognized in the art that it is desirable to provide and utilize polyolefins having bimodal molecular weight distribution. Such distribution is normally shown by a graph of the molecular weight distribution, as determined, for example, by gel permeation chromatography. Generally, the molecular weight distribution is defined by a parameter, known as the dispersion index D, which is the ratio between the weight average molecular weight (Mw) and the number average molecular weight (Mn). The dispersion index constitutes a measure of the breath of the molecular weight distribution. For most applications, the dispersion index varies between 10 and 30.
A truly bimodal distribution curve of a polymer exhibits two peaks which may have substantially equal amplification. In this instance, the polymer composition has substantial amounts of polymer forming a high and a low molecular weight distribution. Alternatively, the resultant molecular weight distribution curve may exhibit a single major peak having a shoulder or smaller second peak on one side of the major peak. Such a curve is provided from polymer compositions having one major polymer distribution with a minor amount of a second polymer distribution. This latter material sometimes designated as being mono-modal.
The manufacture of bimodal polyethylene is known in the art. One method to achieve a bimodal distribution uses two active catalyst species which provide two different catalytic properties and establish two different active sites. Those two sites in turn catalyze two distinct reactions for the production of the two polymers to enable the bimodal distribution to be achieved. In another method, as has been known for many years and exemplified by EP-A-0057420, the commercial production of bimodal high-density polyethylene is carried out by a two step process, using two reactors in series. In the two step process, the process conditions and the catalyst can be optimized in order to provide a high efficiency and yield for each step in the overall process.
In WO-A-95/10548 and WO-A-95/11930, it was proposed to use a Ziegler-Natta catalyst to produce polyethylene having a bimodal molecular weight distribution in a two-stage polymerization process in two liquid full loop reactors in series. In the polymerization process, a comonomer is fed into the first reactor and the high and low molecular weight polymers are produced in the first and second reactors respectively. The introduction of comonomer into the first reactor leads to the incorporation of the comonomer into the polymer chains in turn leading to the relatively high molecular weight fraction being formed in the first reactor. In contrast, no comonomer is deliberately introduced into the second reactor and instead a higher concentration of hydrogen than used in the first reactor is present in the second reactor to enable the low molecular weight fraction to be formed therein.
These prior processes suffer from the technical disadvantages that some unreacted comonomer can pass through from the first reactor to the second reactor, thereby reacting with the ethylene monomer therein leading to an increase in the molecular weight of the fraction produced in the second reactor. This in turn can deteriorate the bimodality of the molecular weight distribution leading to a reduction in the mechanical properties of the resultant polymer product. Further, such two-step processes require maintaining and controlling two separate reactors, which is more labor intensive and can, thereby, increase production costs.
More recently, U.S. Pat. No. 5,714,424 directs one to utilize a catalyst that is a monolithic multi-component composite particle. The composite is formed by physically blending at least two distinct supported catalyst components (each having the capability of producing polymer with distinctly different melt index values under the same polymerization conditions) and forming a combined monolithic particulate catalyst material having each of the supported catalyst components therein. The composite requires initially forming each of the distinct support catalysts to be used and then further processing these catalysts into the final monolithic form. Such composite catalysts may merely cause a broadening of the molecular weight distribution, as exhibited by a molecular weight distribution curve having a single peak and a broad range of differing molecular weight material. Due to differing catalytic activity of each individual catalyst forming a part of the composite particles, any bimodal distribution would be small and would not necessarily provide a polymer having the capacity to exhibit good processability while providing a tough, resilient product.
In addition to realization of the desired product properties, other factors influence the efficiency of a catalyst system, such as the activity of the catalyst system, that is to say the amount of catalyst required for economic conversion of a given amount of olefin, the product conversion per unit time and the product yield.
An additional factor to be considered is the ability to utilize the catalyst in a heterogeneous catalyst system. The advantages of a heterogeneous catalyst system are more fully realized in a slurry polymerization process, of which the Phillips Process is but one example. More specifically, slurry polymerizations are often conducted in a reactor wherein monomer, catalysts, and diluent are continuously fed into the reactor. The solid polymer that is produced is not dissolved in the diluent but is allowed to settle out before being periodically withdrawn from the reactor. In this kind of polymerization, factors other than activity and selectivity, which are always present in solution processes, become of paramount importance.
For example, in the slurry process it is desired to have a supported catalyst that produces relatively high bulk density polymer. If the bulk density is too low, the handling of the solid polymer becomes impractical. It is also an advantage to have the polymer formed as uniform, substantially spherical particles that are relatively free of fines. Although fines can have a sufficiently high bulk density, they also do not settle as well as larger particles and they present additional handling problems with the later processing of such polymer xe2x80x9cfluffxe2x80x9d.
Furthermore, slurry polymerization processes differ in other fundamental ways from typical solution polymerization processes. Solution polymerization is normally conducted at high reaction temperatures ( greater than 130xc2x0 C.) and pressure ( greater than 450 psi) which often results in lower molecular weight polymers. The lower molecular weight is attributed to the rapid chain-termination rates under such reaction conditions. Although lowering the reaction temperature and/or pressure, or changing molecular structure of the catalyst in a solution process may produce higher molecular weight polymers, it becomes impractical to process the resulting high molecular weight polymers in the downstream equipment due to the high solution viscosity.
In contrast, a slurry reaction process overcomes many of the above disadvantages by simply operating at lower temperature ( less than 110xc2x0 C.). As a result, polymer with a uniform particle size and morphology can be routinely obtained. It is also advantageous to carry out slurry reactions with sufficiently high polymerization efficiencies such that residues from the polymerization catalysts do not have to be removed from the resulting polymers.
Examples of materials which are useful as a catalyst support component are described in WO97/48743 directed to spray-dried agglomerates of silica gel of controlled morphology and in U.S. Pat. Nos. 5,395,808; 5,569,634; 5,403,799; 5,403,809 and EP 490,226 directed to formation of particles of bound clay by spray drying.
Supported catalyst systems are described in U.S. Pat. No. 5,633,419, which describes the use of spray-dried silica gel as a support for Ziegler-Natta catalyst systems; U.S. Pat. No. 5,362,825 directed to supported Ziegler-Natta catalysts formed by contacting a pillared clay material with a Ziegler-Natta catalyst composition; U.S. Pat. No. 5,807,800 directed to a supported metallocene catalyst formed by contacting a particulate support with a formed stereospecific metallocene ligand; U.S. Pat. No. 5,238,892 directed to use of undehydrated silica as support for metallocene and activator; and U.S. Pat. No. 5,308,811 directed to formation of supported metallocene-type transition metal compound by contacting it with a clay and an organoaluminum compound.
WO 0125149 A2 discloses a composition comprising an acid treated cation exchanging layered substrate material dispersed in silica gel as a support for a metallocene polymerization catalyst. Acidification is accomplished using a Bronsted acid such as sulfuric acid or an acidified amine, e.g., ammonium sulfate in a mixture with alkaline metal silicate such that the latter precipitates as silica hydrogel. The resulting slurry is dried, e.g., spray dried, and contacted with a metallocene catalyst. Preferably the layered silicate material is fully acid exchanged.
WO 0149747A1 discloses a supported catalyst composition comprising an organoaluminum compound, an organometal compound and an oxide matrix support wherein the latter is a mixture of an oxide precursor compound such as a silica source and a substantially decomposed (exfoliated) layered mineral such as a clay. Decomposition of the clay is achieved, for example, by solvent digestion in a strong acidic and basic medium at elevated temperatures combined with high energy or high shear mixing to product a colloidal suspension. Decomposition (exfoliation) converts the material to its residual mineral components and is said to be complete when the layered mineral no longer has its original layered structure.
WO 0142320 discloses a clay or expanded clay useful as a polymerization catalyst support. The support comprises the reaction product of the clay or expanded clay with an organometallic, or organometalloid, compound in order to reduce, cap or remove residual hydroxyl or other polar functionality of the clay and replace such groups with the organometallic compound. An organometallic or organometalloid derivative is bound to the support through the support oxygen or other polar functionality. Prior to reaction with the organometallic compound, the clay can be ion exchanged to replace at least a portion of alkali or alkali earth metal cations, e.g. sodium or magnesium, originally present in the clay. The chemically modified clay may be calcined either before or after treatment with the organometallic compound; prior treatment is preferred. The organometallic or organometalloid compound contains Mg, Zn or boron, preferably Zn, and the organic group preferably is a C1-C10 alkyl.
The teachings of intercalated clays as support materials for catalytic compositions include: U.S. Pat. No. 5,753,577 (directed to a polymerization catalyst comprising a metallocene compound, a co-catalyst such as proton acids, ionized compounds, Lewis acids and Lewis acidic compounds, and a clay mineral); U.S. Pat. No. 5,399,636 (directed to a composition comprising a bridged metallocene which is chemically bonded to an inorganic moiety such as clay or silica); EP 849,292 (directed to an olefin polymerization catalyst consisting essentially of a metallocene compound, a modified clay compound, and an organoaluminum compound); U.S. Pat. No. 5,807,938 (directed to an olefin polymerization catalyst obtained by contacting a metallocene compound, an organometallic compound, and a solid component comprising a carrier and an ionized ionic compound capable of forming a stable anion on reaction with the metallocene compound); U.S. Pat. No. 5,830,820 and EP 881,232 (directed to an olefin polymerization catalyst comprising a metallocene compound, and an organoaluminum compound and a clay mineral which has been modified with a compound capable of introducing a cation into the layer interspaces of the clay); EP 849,288 (discloses an olefin polymerization catalyst consisting essentially of metallocene compound, an organoaluminum compound, and a clay compound that has been modified with a proton acid); and U.S. Pat. No. 4,761,391 (directed to delaminated clays whose x-ray diffraction patterns do not contain a distinct first order reflection. These clays are made by reacting swelling clays with a pillaring agent. The ratio of clay to pillaring agents is disclosed to be between about 0.1 and about 10. To obtain the delaminated clay, a suspension of swelling clay, having the proper morphology, e.g., colloidal particle size, is mixed with a solution or a suspension of the pillaring agent at the ratios described above.)
Additional patents that disclose intercalated clays are U.S. Pat. Nos. 4,375,406; 4,629,712; and 4,637,992. Additional patents that disclose pillared clays include U.S. Pat. Nos. 4,995,964 and 5,250,277.
U.S. Ser. No. 4/431,803 filed on Nov. 1, 1999, by Keng-Yu Shih discloses the use of silica agglomerates as a support for transition metal catalyst systems employing specifically controlled (e.g., very low) amounts of non-abstracting aluminum alkyl activators.
U.S. Ser. No. 09/431,771, filed on Nov. 1, 1999, by Keng-Yu Shih et al. discloses a coordination catalyst system comprising a bidentate or tridentate pre-catalyst transition metal compound, at least one support-activator, e.g., spray dried silica/clay agglomerate, and optionally an organometallic compound and methods for their preparation.
U.S. Ser. No. 09/432,008, filed on Nov. 1, 1999, by Keng-Yu Shih et al. discloses a coordination catalyst system comprising a metallocene or constrained geometry pre-catalyst transition metal compound, at least one support-activator, e.g., spray dried silica/clay agglomerate, and optionally an organometallic compound and methods for their preparation.
U.S. provisional application Serial No. 60/287,607, filed on Apr. 30, 2001, discloses a process for forming a catalyst composition comprising substantially simultaneously contacting at least one bidentate ligand compound or at least one tridentate ligand compound or mixtures thereof with a transition metal compound and with a support-activator agglomerate comprising (A) at least one inorganic oxide component, and (B) at least one ion-containing layered component. The reference further is directed to the resultant catalyst composition for which the support-activator agglomerate functions as the activator for the catalyst system.
U.S. provisional application Serial No. 60/287,602, filed on Apr. 30, 2001, discloses a catalyst composition composed of a support-activator agglomerate comprising (A) at least one inorganic oxide component, and (B) at least one ion-containing layered component. The agglomerate provides a support-activator agglomerate for a combination of catalysts comprising at least one metallocene catalyst and at least one coordination catalyst of a bidentate or tridentate pre-catalyst transition metal compound.
U.S. provisional application Serial No. 60/287,617, filed on Apr. 30, 2001, discloses a process for forming a catalyst composition comprising substantially simultaneously contacting at least one bidentate ligand compound or at least one tridentate ligand compound or mixtures thereof with a transition metal compound and with a support-agglomerate comprising (A) at least one inorganic oxide component, and (B) at least one ion-containing layered component and the agglomerate has chromium atoms immobilized thereto. The reference is further directed to the resultant catalyst composition for which the support-agglomerate functions as the activator for the catalyst system.
U.S. provisional application Serial No. 60/287,600, filed on Apr. 30, 2001, discloses a catalyst composition composed of a support-agglomerate comprising (A) at least one inorganic oxide component, and (B) at least one ion-containing layered component and the agglomerate has chromium atoms immobilized thereon. The agglomerate provides a support for at least one coordination pre-catalyst comprising a transition metal bidentate or tridentate ligand containing complex.
In addition, the following U.S. patent applications having Ser. No. 10/120,291; Ser. No. 10/120,317; Ser. No. 10/120,331; Ser. No. 10/120,310; Ser. No. 10/120,314 are concurrently filed with the subject application. The teachings of each of the above provisional and concurrently filed applications are incorporated herein in its entirety by reference.
Thus, there has been a continuing search to develop catalyst systems that demonstrate high catalyst activity, are readily formed in an inexpensive and efficient manner, and allows the polymerization process to be conducted as a cost effective one-step polymerization reaction. Further, there has also been a particular need to discover catalysts that can provide polymers, in particular polyolefins (e.g., polyethylene) having bimodal molecular weight distribution in a single step reaction and to polymers resulting therefrom which can be readily processed to form products exhibiting toughness and resiliency.
The present invention is directed to a unique and novel supported chromium catalyst composition.
Further, the present invention is directed to polymerization processes, which utilize the present catalyst to produce polymers, in particular polyolefins, having bimodal molecular weight distribution.
Still further, the present invention is directed to a polymerization process, which utilizes the present catalyst to produce bimodal polymer product in a single polymerization step.
More specifically, the present invention is directed to supported chromium catalysts wherein the chromium atom is immobilized to agglomerates formed from at least two components, namely (A) at least one inorganic oxide component and (B) at least one ion-containing layered component, as fully described herein below.
Moreover, the supported chromium catalyst can be used in combination with other catalyst species, such as metallocene, constrained geometry, bidentate or tridentate ligand containing compounds and mixtures thereof wherein the catalyst specie is present on a support in the form of an agglomerates formed from at least two components, namely (A) at least one inorganic oxide component and (B) at least one ion-containing layered component, which may optionally have chromium atoms bonded thereto.